The broad and long-term objectives of the research program are to uncover the molecular mechanism of Jak/STAT-mediated signal transduction and transcription activation. STATs are so named since they are signal transducers in the cytoplasm and activators of transcription in the nucleus. Upon receptor dimerization induced by ligand binding, Jak or receptor tyrosine kinases are activated and they in turn phosphorylate tyrosines on the receptors. The SH2-containing STAT molecules are recruited to the cell surface and become tyrosine phosphorylated by Jak or receptor kinases. They subsequently homo- or hetero-dimerize and translocate into the nucleus, where they bind to specific DNA targets and direct specific transcription initiation. The Jak/STAT pathway is involved in the signaling process of virtually every cytokine and growth factor. Dysregulation of this pathway leads to constitutively activated STAT proteins and the subsequent up-regulation of the genes of apoptosis inhibitors and cell cycle regulators. Constitutively activated STATs have been linked to numerous cancer cell lines and solid tumors including leukemia and breast cancer. Thus they have become important targets for cancer drug discovery. The major focus of this research is to elucidate the molecular mechanism underlying STAT nuclear localization at atomic resolution. The specific aims of the proposed research include (1) to determine the crystal structure of an unphosphorylated full-length STAT; (2) to study the physiological significance of the observed unphosphorylated STAT dimer structure using biochemical, biophysical, and cell biological tools; and (3) to study STAT1:NPI1 interaction and its role in STAT1 nuclear localization using biochemical, structural, and cell biological tools.